In a process of producing a semiconductor device, a film to be treated (for example, an insulating film or an electroconductive film) is generally formed on a base substrate. An etching mask is formed on the film, and a pattern having predetermined dimension and shape is formed by etching in the film to be treated. The above procedure is repeated a plurality of times.
In the step of forming a pattern in the film to be treated, a lithography technique is generally used wherein the so-called exposure step, development treatment step and the like are carried out using a photosensitive material called a photoresist (hereinafter referred to simply as “resist”).
When the lithography technique is used, a pattern of the film to be treated is generally formed according to the following procedure. Specifically, a resist film is formed by coating on a film to be treated, and the resist film in its predetermined region is exposed imagewise through an exposure mask (=reticle). Next, the resist film after the exposure is developed, and the exposed or unexposed part is removed to form a resist pattern. A pattern of the film to be treated is then formed, for example, by drying etching using this resist pattern as a mask.
In recent years, when a resist film is exposed, short wavelength exposure light should be used, for example, from the viewpoints of improved resolution and throughput. For example, light sources which emit ultraviolet light, for example, KrF excimer laser or ArF excimer laser, are used.
Even when exposure is carried out using the short wavelength light, the attainable minimum line width is about 90 nm. On the other hand, there is an increasing demand for an increase in resolution of the pattern, and, in the future, it is not hard to anticipate that a pattern having a higher level of fineness is required. What is most important to form a pattern having a higher level of fineness than 90 nm is to develop an exposure system and a resist compatible with the exposure system. Points which are generally important for the development of the exposure system are, for example, the use of shorter wavelength as the light source wavelength, for example, F2 excimer laser, EUV (extreme ultraviolet light), electron beams, X rays, and soft X rays, and an increase in numerical aperture (NA) of lenses. However, the adoption of a shorter wavelength as the light source wavelength requires an expensive new exposure system, and increasing the numerical aperture involves a problem that, the resolution and the focal depth width are in a trade-off relationship. Accordingly, even when the resolution is enhance, the focal depth width is disadvantageously lowered.
In recent years, a liquid immersion exposure method (liquid immersion lithography) has been proposed as a lithography technique which can solve the above problem. In this method, a liquid refractive index medium (a refractive index liquid, an immersion liquid) such as pure water or a fluorine-type inert liquid is interposed in a predetermined thickness in a part between a lens and a resist film on a substrate in the step of exposure, at least on the resist film. In this method, by virtue of the replacement of an exposure light path space which has hitherto been filled with an inert gas such as air or nitrogen, with a liquid having a larger refractive index (n), for example, pure water, even when a light source having the same exposure wavelength as in the case of the inert gas is used, as with the case where a shorter wavelength light source or a higher NA lens is used, a high resolution can be realized and, at the same time, the focal depth width is not reduced. The liquid immersion exposure has drawn attractive attention because, the use of the liquid immersion exposure can realize the formation of a resist pattern at a lower cost with a higher level of resolution and a better focal depth using a lens mounted on the existing apparatus. Even the liquid immersion exposure cannot realize a higher resolution pattern than the resolution limit 40 nm. Under such circumstances, various proposals have been made for realizing a higher resolution pattern.
Non-patent document 1 discloses fining of a pattern by two times exposure. In this method, a resist pattern is formed on a hard mask as a first layer by a photoresist process. Thereafter, the resist pattern is transferred onto the hard mask as the first layer by etching, and the resist is then removed. A resist pattern is then formed by a second photoresist process. The hard mask as the first layer after above transfer is again used, and the pattern is transferred by etching to the hard mask as the lowermost layer. The above two-times exposure method can realize the transfer of a superfine pattern. In this method, however, since the photoresist process is repeated twice, the time necessary for the process is increased. Accordingly, the photoresist process is disadvantageous from the viewpoint of cost. Further, the exposure involved in the photoresist process is repeated twice, two types of reticles are necessary. This requires highly accurate registration of a transferred image obtained by the transfer of a first reticle image with a transferred image obtained by the transfer of a second reticle image, and, thus, there is room for an improvement in simplicity and mass productivity.
Non-patent document 2 discloses a method for forming a superfine pattern in which a spacer is formed by a chemical vapor deposition method (hereinafter referred as “CVD”) on the side wall of a patterned template formed on a film to be treated, and is used as a mask for conducting etching. This method is carried out according to the following procedure. A resist pattern is formed by a photoresist process on a previously formed template layer. An image is transferred on the template layer using the resist pattern as a mask to form convexes (a patterned template layer). Further, a spacer is formed on the side wall of the convexes by CVD. Thereafter, the convexes are removed to allow the spacer to stay as a pattern. In this case, when the patterned template layer is in a line form, two patterns of the spacer are formed for each original one line. Specifically, the pattern can be doubled. The film to be treated is treated using the doubled pattern as an etching mask to form a superfine pattern. In this non-patent document 2, a 22 nm-superfine pattern can be successfully formed by this method. In the CVD method for realizing this method, a considerably lot of time is required for the process, and, thus, the method should be significantly improved for improved mass productivity.
[Non-patent document 1] M. Dusa, et al., “Pitch doubling through dual patterning lithography challenges in integration and Lithography budgets”, Proc. SPIE Vol 6520, 65200G, (2007)
[Non-patent document 2] Chris Bencher, Nanochip technology journal, issue of two 2007